L-Menthol has a unique refreshing taste, a minty odour and a pronounced cooling effect on the skin and mucosa. It is used, for example, in oral care, in cosmetic and pharmaceutical preparations, in tobacco and in confectionery, as described, for example, in Perfumer & Flavorist, Vol. 13, October-November 1988, p. 37.
L-Menthol is the main constituent of the peppermint oils from Mentha arvensis (content: 70 to 80%) and Mentha piperita (content: 50 to 60%). L-Menthol is obtained from the crude peppermint oil by crystallisation. Depending on the crystallisation method and the starting material, the crystals differ in terms of taste and also in terms of the size and shape of the crystals (Perfumer & Flavorist; Vol. 22, November-December 1997, p. 1). A small residual amount of liquid peppermint oil adheres to these menthol crystals obtained from peppermint oils (the content of l-menthol is conventionally not more than 99.2 wt. %) and inhibits caking or clumping of the menthol crystals, but also has a marked influence on the sensory profile.
Many processes for the preparation of synthetic menthol are known. An economic process for the preparation of synthetic l-menthol uses thymol, for example, as starting material. From the eight stereoisomeric menthols formed by hydrogenation, l-menthol is obtained via a plurality of process steps in a chemical purity of >99% and an enantiomeric purity of >99% (e.g. in Bauer, Garbe, Surburg, Common Fragrance and Flavor Materials, 4th Ed., Wiley-VCH, Weinheim 2001, p. 52-55). The l-menthol resulting from this process is distinguished in sensory terms by its purity and intensity.
Menthol, in particular l-menthol, is commercially available in various solid forms; powders, crystals, solidified distillate, flakes and pressed articles, for example, are conventional.
Synthetic l-menthol (having a melting point of from 42 to 43° C.) prepared and crystallised according to the processes from DE-A 2109456, DE-A 2530481 and EP 0 909 205 is available commercially in the form of white crystals or in the form of pressed articles (pellets) as well as in the form of a solidified distillate (Symrise GmbH & Co. KG, Holzminden).
Solid forms of menthol, in particular of peppermint-oil-free menthol, known hitherto form clumps after a prolonged time during storage, which impairs their pourability and handling ability.
In U.S. Pat. No. 3,064,311, l-menthol in flake form is described. For its preparation, distilled l-menthol is melted, and a thin, molten film layer is applied to a supercooled surface. The solidified l-menthol film is broken into small pieces. The product of this process is a brittle l-menthol flake which has a thickness of from 0.125 to 1.25 mm and a spread of from 3 to 25 mm. The flaked l-menthol so prepared exhibits adhesion and clumping of the flakes after 24 hours, as described in the US specification.
WO 03/101924 describes menthol pressed articles which have a content of alpha-menthol of greater than or equal to 70 wt. % and exhibit a comparatively very low tendency to caking or clumping. However, even these pressed articles gradually clump and cake together when stored for prolonged periods of over three months.
JP 08-020549 relates to l-menthol-containing powder comprising about 90 wt. % synthetic l-menthol (particle size from 50 to 200 μm) and about 10 wt. % silica gel (particle size less than 10 μm, preferably in the range from 2 to 5 μm). This product (bulk height: 50 cm) did not exhibit caking after one month's storage. For the preparation of the product, a surfactant (e.g. 2 wt. % decaglyceryl monolaurate, based on the total amount of menthol and silica gel) was first introduced into water at a temperature above the melting point of l-menthol (42° C.). Silica gel and molten menthol, or menthol on silica gel, were then introduced in succession into the mixture of water and surfactant at 50° C. After stirring for a short time, this mixture was cooled with ice-water, washed with water, filtered off and dried.
Our own tests have shown, however, that the product prepared according to JP 08-020549 exhibits a water content of up to 3 wt. % after drying. As already mentioned in the introduction to JP 08-020549, moisture leads to a lowering of the melting point of menthol and to a certain tendency to caking; the water content is accordingly disadvantageous. Furthermore, the residual content of surfactant in the described powdered, menthol-containing product is undesirable. In addition, the high content of silica gel in the disclosed menthol-silica gel powder is disadvantageous; inter alia, the silica gel imparts an unpleasant sensory impression in the mouth.